Might go buy a 1999 Polaris 500 for $500 obo. He said "It blew the head gasket and it needs a piston cylinder is ready to be put back on and head is true". Anything else I should look for on these quads, I'll probably just bore out the motor and rebuild it.

 

Look at the cam,bad on these models about ground down exhaust lobes. Always expect the unexpected on an atv that's not running and don't believe anything that anyone says unless it's your mama.. Here's one example on a ground down cam. Bad Cam Pic - ATVConnection.com ATV Enthusiast Community